This invention concerns a technique for forming a fragmented permeable mass of particles in an in situ oil shale retort and in particular relates to explosive expansion of pillars within voids excavated within the retort site before explosive expansion of adjacent formation containing oil shale.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" are used in the industry is, in fact, a misnomer, it is neither shale nor does it contain oil, it is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the treated shale remains in place, reducing the change of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents such as U.S. Pat. Nos. 3,661,423; 4,043,597; 4,043,598; and 4,153,298, as well as pending applications including U.S. patent application Ser. No. 929,250, filed July 31, 1978, by Thomas E. Ricketts, now U.S. Pat. No. 4,192,554 and U.S. patent application Ser. No. 070,319, filed Aug. 27, 1979, by Chang Yul Cha, entitled TWO-LEVEL, HORIZONTAL FREE FACE MINING SYSTEM FOR IN SITU OIL SHALE RETORTS. Each of these applications and patents is assigned to Occidental Oil Shale, Inc., assignee of this application, and each is incorporated herein by this reference.
These patents and applications describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort, or merely as a retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbons, and a residual carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products together with water produced in or added to the retort collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition and any gaseous retort inlet mixture that does not take part in the combustion process.
U.S. Pat. Nos. 4,043,597 and 4,043,598, and application Ser. No. 929,250, disclose methods for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to such a method a plurality of vertically spaced apart voids of similar horizontal cross section are initially excavated one above another within the retort site. A plurality of vertically spaced apart zones of unfragmented formation are temporarily left between the voids. Explosive is placed in each of the unfragmented zones and detonated to explosively expand each unfragmented zone upwardly and/or downwardly towards the void or voids above and/or below it to form a fragmented mass having an average void volume about equal to the void volume of the initial voids. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
U.S. patent application Ser. No. 070,319 discloses a method for explosively expanding formation containing oil shale towards a horizontal free face to form a fragmented mass in an in situ oil shale retort. According to such a method, a void having a horizontal cross section similar to the horizontal cross section of the retort being formed is initially excavated. A plurality of vertically spaced apart zones of unfragmented formation are left above the void. Explosive is placed in each of the unfragmented zones and detonated for explosively expanding such zones towards the void to form a fragmented mass in the retort having an average void volume about equal to the void volume of the initial void. The overlying zones can be expanded towards the void in a single round or a plurality of rounds. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
U.S. Pat. No. 4,153,298 describes a method for forming a retort by excavating at least one horizontally extending void adjacent a zone of unfragmented formation to be expanded. At least one support pillar of unfragmented formation is left in the void for supporting overburden. Explosive is placed in the zone of unfragmented formation and in such a support pillar. Explosive in such a pillar and in the zone of unfragmented formation is detonated in a single round of explosions with a time delay between detonation of explosive in such a pillar and detonation of explosive in the zone of unfragmented formation for first expanding such a pillar toward the void and then expanding unfragmented formation toward the void. The time delay is sufficient for creation of a free face at the juncture of such a pillar and the zone of unfragmented formation. The time delay is short enough that explosive in the zone of unfragmented formation is detonated before particles formed by explosive expansion of the pillars have come to rest on the floor of the void.
It is desirable to have a generally uniformly distributed void fraction in the fragmented mass so that there is generally uniform permeability. Thus, oxygen supplying gas and combustion gas can flow reasonably uniformly through the fragmented mass during retorting operations. A fragmented mass having generally uniform permeability avoids bypassing portions of the fragmented mass by retorting gas as can occur if there is gas channelling through the mass due to non-uniform permeability.
Is is also desirable that the fragmented permeable mass in a retort have a reasonably flat upper surface near or in contact with overlying unfragmented formation. It is preferable to maintain a resonably flat and horizontal combustion zone advancing through the fragmented mass in the retort during retorting. Establishment of such a flat combustion zone is significantly simplified when the upper surface of the fragmented mass is reasonably flat and horizontal.
Thus, in summary, it is desirable to provide a fragmented mass in a retort having reasonable uniformity of particle size and void fraction distribution and with a reasonably level upper surface.
It was found upon forming a retort generally in accordance with the description in U.S. patent application Ser. No. 929,250 that the fragmented mass of particles in the retort did not completely fill the retort cavity, that is, a void space remained between the upper surface of the fragmented mass and overlying unfragmented formation. In addition, a tendency was noted for the fragmented mass in the retort to form a mound, that is, the upper surface of the fragmented mass was relatively high near the middle of the retort and was relatively low nearer the side boundaries. It is desirable to avoid such mounding in an in situ oil shale retort.
It was also found that an appreciable quantity of formation particles from the fragmented mass entered the access drifts to the voids excavated within the boundaries of the retort site. It is believed that this tended to increase void fraction in adjacent portions of the fragmented mass and may have aggravated the tendency to form a mound in the fragmented mass. It is believed that at least a portion of the fragmented formation in the access drifts was from a zone of formation explosively expanded from above the respective void. Such formation may have moved into such a drift under the influence of gravity subsequent to explosive expansion. Such movement could increase void fraction in adjacent portions of the fragmented mass. It is therefore desirable to minimize the effect of access drifts through side boundaries of the retort.